1. Field of the Invention
The present invention relates to the field of thin films and their methods of manufacturing and more specifically, to composite silicon/tungsten-silicide thin films and their methods of fabrication.
2. Discussion of Related Art
As features of VLSI circuits continue to shrink, the necessity of decreasing the resistance associated with interconnection paths and gate electrodes becomes ever more pressing. Polysilicon has been widely used for many years in the formation of gate electrodes and interconnects. Polysilicon, however, has been somewhat limited by its high bulk resistivity of 0.7 mohm-cm. Polycide films, which consist of a low resistance silicide formed on top of a doped polysilicon film, have been used in place of polysilicon films in an attempt to further decrease the resistance of gate electrodes and interconnects to thereby create faster and lower power consuming integrated circuits.
A polycide film comprising a tungsten-silicide layer formed on a doped polysilicon layer has been widely used as gate electrodes and interconnections in many metal oxide semiconductor (MOS) processes. In the fabrication of a doped polysilicon/tungsten-silicide gate electrode, as shown in FIG. 1a, a doped polysilicon layer 104 of approximately 1000 angstroms (.ANG.) is blanket deposited onto a gate dielectric layer 102 formed over the surface of a wafer or substrate 100. A tungsten-silicide layer 106, of the same order of magnitude in thickness as polysilicon layer 104 is then deposited over doped polysilicon layer 104 to form a "polycide" stack 108. Tungsten-silicide layer 106 can be formed by chemical vapor deposition (CVD) utilizing a reduction reaction of monosilane (SiH.sub.4) and tungsten hexafluorine (WF.sub.6). Such a deposition technique forms a good quality film having a uniform thickness across the surface of the wafer and a relatively smooth top surface. Unfortunately, however, utilizing a SiH.sub.4 reduction of WF.sub.6 to form tungsten-silicide layer 106 incorporates a substantial amount of fluorine atoms into the tungsten-silicide layer which can later damage or destroy fabricated devices.
In order to reduce fluorine incorporation, tungsten-silicide layer 110 can be formed by CVD utilizing a reduction reaction of dichlorosilane (DCS or SiH.sub.2 Cl.sub.2) and tungsten hexafluorine (WF.sub.6). Although such a DCS based reaction substantially eliminates fluorine incorporation, the film is of low quality because dopant impurities in polysilicon layer 104 prevent good nucleation from occurring. As a result, as shown in FIG. 1b, the film forms nonuniformly over the surface of the wafer and has an unacceptably rough surface. Such a polycide film 112 is unacceptable for VLSI manufacturing. Another problem associated with forming a DCS based tungsten-silicide layer on a doped polysilicon layer is that such a deposition technique can form a tungsten rich interface which is known to negatively impact device performance and cause reliability problems such as electromigration and high film stress. The presence of polysilicon dopants at the interface can also lead to the formation of an undesired tungsten rich interface.
It has been discovered that in order to produce a tungsten-silicide film of good quality by a DCS reduction of WF.sub.6, it is necessary to form a thin, at least 150 .ANG., undoped polysilicon capping layer 114 on doped polysilicon layer 104. Undoped polysilicon capping layer 114 provides a barrier between the dopant impurities in doped polysilicon layer 104 and DCS based tungsten-silicide film 116. In this way, a DCS based tungsten-silicide film 116 can be formed with good uniformity and a substantially smooth surface across a wafer. Unfortunately, however, such a technique is only viable for polycide films 120 having a thickness greater than 1000 .ANG.. For thinner films, less than 1000 .ANG., the undoped capping layer 114, which must still be at least 150 .ANG. to produce a sufficient barrier, comprises too much of the total film thickness and results in a substantial reduction in the total dopant concentration of the polycide film 120 which in turn results in an increase in total resistance. Attempts to further increase the doping concentration level of the polysilicon layer 104 to offset capping layer 114 results in dopant diffusion into gate oxide 102 and consequently to device performance degradation and damage.
Thus, what is desired is a method of forming a thin low resistance polycide film having a uniform thickness and a smooth top surface which does not incorporate an undesirable amount of fluorine, and which does not have a tungsten rich interface.